What effects do open marriages have on kids?

My brother and sister-in-law have an open marriage, meaning they can each have relationships on the side. They each have a few girlfriends, but are still married and in love and say that their marriage has never been stronger. The research shows that couples in open marriages have longer lasting, more meaningful relationships than those in monogamous marriages, but what effects does this have on kids? My brother and sister-in-law have a 3 year old son, and he is fine now, but will this somehow mess him up in the future?